TOMO 2 - Volcanismo y Magmatismo THE ROLE OF CONTINENTAL CRUST DURING MAGMA GENESIS BENEATH HURD PENINSULA (SOUTH SHETLAND ISLANDS, ANTARCTICA) – CLUES FROM MAGMATIC DYKES S. Kraus (Instituto Antártico Chileno, Punta Arenas, [email protected]) H. Miller (Dept. of Earth & Environm. Sci., Univ. of Munich, Germany, [email protected]) INTRODUCTION Hurd Peninsula (62° 40’ S / 60° 22’ W) is located in southern Livingston Island, the second largest of the South Shetland Islands (Fig. 1). It is bordered by South Bay in the NW and False Bay in the SE and comprises about 27 km2. Three main lithological sequences crop out there: the sedimentary, turbiditic Miers Bluff Formation (MBF) in the stratigraphically lowermost position, unconformably overlain by the Moores Peak Breccia and, uppermost, the volcanic rocks of the magmatic arc (Mount Bowles Formation). The investigation area is located in the northwestern part of Hurd Peninsula and the dykes are hosted without exception by the Miers Bluff Formation. Major goals were the identification of tectonic events as reflected by the orientation of the dykes and by joint systems, the determination of the magma sources as well as possible changes, and relationships between tectonic and magmatic events. Ar-Ar age determinations carried out on plagioclase separates indicate that the dykes intruded the different directions (first and second order dextral and sinistral shear systems) correlated with the folding axis (24°, evident in the sedimentary host rocks) from Danian on, most of them during the Lutetian. Six different intrusive events can be distinguished on Hurd Peninsula. Fig. 1: Geography of the northern Antarctic Peninsula Dykes of the same relative age also share the and the South Shetland Islands and their location in Antarctica (inset). same strike direction, indicating that slight changes 483 XI CONGRESO GEOLOGICO CHILENO of the overall tectonic parameters led to different preferred directions, but within the same general stress field [KRAUS & MILLER 2003a]. THE GEOCHEMISTRY OF THE DYKES Most of the dykes investigated in other parts of the South Shetland Islands correspond to a typical subduction-related transitional to calc-alkaline suite (Fig. 2), ranging from basalts to rhyolites [KRAUS & MILLER 2003b]. In contrast, the oldest dykes on Hurd Peninsula show a shoshonitic composition possibly related to an early stage extensional crustal regime. The magmatic affinity very soon became tholeiitic and only the two last intrusive events at Hurd Peninsula show calc-alkaline characteristics. Tholeiites have been reported as being typical for young immature arcs, moreover for more mature arcs in settings closest to the trench [e.g. TATSUMI & EGGINS 1995]. We assume that these dykes intruded during early stages of arc activity in that area, the magma travelling through a still unstretched crust and consequently showing both tholeiitic characteristics and the influence of conti-nental crust assimilation. The six different intrusive events indicated on Hurd Peninsula by tectonic data are also confirmed by geochemistry using trace and rare earth element ratios (e.g. Nb/Y, Zr/Hf, Y/Ho, Ce/Pb) and Mg#. The geochemical and isotopic data show that four different magma sources contributed to arc magma genesis: subducted sediments, fluids derived from the subducted plate, the mantle wedge and the continental crust underneath the South Shetland Islands [KRAUS 2005]. THE FINGERPRINT OF THE CONTINENTAL CRUST From Sr, Nd and Pb isotope data alone, it is difficult or impossible to distinguish be-tween Fig. 2: Zr plotted vs. Y to reveal the magmatic affinity [MCLEAN & BARRETT 1993]. Element concentrations are sediment influx and crustal contamination in active given in µg/g. Primitive mantle values from MCDONOUGH & SUN [1995], N-MORB values from SUN continental margin settings. However, crustal & MCDONOUGH [1989], upper continental crust (UCC) values from RUDNICK & GAO [2003]. contamination usually occurs during shallow level 484 TOMO 2 - Volcanismo y Magmatismo differentiation. Conse-quently, if such a process takes place, the most differentiated rocks should also be the most contaminated. Thus, plotting isotope data (e.g. eNd) versus some differentiation factor (e.g. Mg#) may yield a correlation in case that contamination occurred. A correlation between eNd and Mg# is obvious for the Hurd Peninsula dykes, but not for dykes from the other areas further to the NE (Fig. 3A). Moreover, they plot roughly according to their relative ages as deduced from field relationships and Ar-Ar data, i.e. the oldest dykes display the lowest eNd and Mg-numbers (Fig. 3B). Therefore, we suppose that particularly the early dykes reflect intrusive events that took place during initial stages of magmatic arc activity in that area. This assumption is consis-tent with a still relatively unstretched continental crust and therefore higher degrees of crustal contamination, leading to lower eNd values and higher incompatible element abun-dances abundances. The aforementioned tholeiitic affinity of most Hurd Peninsula dykes is another argument. The crustal influence then ceased with time (due to an increasingly stretched crust?), and the two youngest events on Hurd Peninsula, that even postdate the dykes from the other areas, plot within the Fig. 3: (A) Crustal contamination as reflected by the correlation of Mg# and εNd in case of Hurd Peninsula. Average 2òm for Nd isotope data is 0.0026 %. (B) Development of εNd through time. The question mark (lower right) indicates the uncertainty of this age, the dark grey area denotes the time of intrusive activity on King George Island. Note that some older dykes from Hurd Peninsula do not plot in this diagram due to missing Nd isotopic data. Digits in brackets refer to the number of analyzed dykes. 485 XI CONGRESO GEOLOGICO CHILENO "uncontaminated" field occupied by dykes from areas further to the NE. Consequently, they are of calc-alkaline, not tholeiitic character. Also low U/Th ratios may reflect crustal contamination. Whereas the Hurd Peninsula dykes in general tend towards low U/Th ratios as compared to the other areas, the two youngest events indeed have the highest U/Th ratios from all Hurd Peninsula dykes, provid-ing further evidence for a gradual decrease of the crustal contamination [KRAUS 2005]. CONCLUSIONS The outlined observations yield strong evidence that crustal contamination beneath Hurd Peninsula was high during initial phases of intrusive activity (Danian), possibly due to a still unstretched continental crust. The crustal influence then decreased gradually and had virtually expired at the time the last two intrusive events intruded (Priabonian). In contrast, dyke formation in areas further northeast (Nelson and King George Island) started not only later but was moreover at no time affected by significant crustal contamination. REFERENCES: KRAUS, S. (2005): Magmatic dyke systems of the South Shetland Islands volcanic arc (West Antarctica): reflections of the geodynamic history. PhD thesis published online (http://edoc.ub.uni-muenchen.de/ archive/00003827/), Munich University Library, pp. 160. KRAUS S. & MILLER H. (2003a): Subduction related dyke systems of the South Shetland Islands, West- Antarctica - tracing geodynamic history combining structural, geochemical and isotopic data. In: FÜTTERER D.K. (ed.): Antarctic contributions to global earth sciences. 9th Int. Symp. Ant. Earth Sci. (ISAES), Terra Nostra, 2003/4, 188-189. KRAUS S. & MILLER H. (2003b): Structural, geochemical, and isotopic studies on magmatic dyke swarms of the South Shetland Islands volcanic arc, West Antarctica - Revealing the geodynamic history. Eos. Trans. AGU, 84(46), Fall Meet. Suppl., Abstract V31C-0952. MACLEAN W.H. & BARRETT T.J. (1993): Lithogeochemical techniques using immobile elements. J. Geo-chem. Expl. ,48, 109-133. MCDONOUGH W.F. & SUN S.-S. (1995): The composition of the Earth. Chem. Geol., 120, 223-253. RUDNICK R.L. & GAO S. (2003): Composition of the continental crust. In: RUDNICK R.L. (ed.): Treatise on geochemistry, Vol. 3. The Crust, Elsevier Ltd., 1-64. SUN S.-S. & MCDONOUGH W.F. (1989): Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In: SAUNDERS A.D. & NORRY M.J. (eds.): Magmatism in the Ocean Basins, Geol. Soc. Spec. Pub., 42, 313-345. TATSUMI Y. & EGGINS S. (1995): Subduction zone magmatism. Blackwell Science, 211 pp., Massachusets. 486 .